People pray for the health of Thailand's King Bhumibol Adulyadej at the Siriraj hospital where he is staying, in Bangkok December 4, 2014. Photo: Reuters

Thai police said on Thursday the chief suspect in a high-profile corruption scandal is the uncle of Princess Srirasmi, who is married to Thailand's heir apparent, Crown Prince Vajiralongkorn.

News about the palace in Thailand is tightly controlled and perceived insults against members of the royal family are often punished with jail. Police had not previously confirmed to Reuters the link between the princess and the suspect.

The corruption investigation has embroiled top police officers, army sergeants, one of Thailand's richest men and seven relatives of the princess, 42, Vajiralongkorn's third wife.

Pongpat Chayaphan, the ex-head of the Central Investigation Bureau (CIB), and several others were charged with extortion, operating gambling dens, accepting bribes and exploiting the monarchy for personal gain.

Pongpat is the princess's uncle on her mother's side, said Lieutenant General Prawut Thawornsiri, spokesman for the police and acting CIB commissioner.

"The ex-CIB chief is the princess's uncle. The princess is his niece," Prawut told Reuters.

Police confirmed on Monday that the crown prince had asked the government to forbid anyone from using Akrapongpreecha, the royal surname given to Srirasmi's family after he married her in 2001.

The corruption scandal has riveted the nation as it prepares to celebrate the birthday of King Bhumibol Adulyadej, who turns 87 on Friday.

The king, who underwent surgery in October to remove his gallbladder, is seen by many Thais as a unifying figure. The crown prince does not yet command the same devotion.

Anxiety over the succession has formed a backdrop to political turbulence in Thailand, where the government was overthrown by a staunchly royalist faction of the military in May.

Three suspects, who until recently shared with the princess the royally bestowed surname Akrapongpreecha, belonged to a gang hired by tycoon Nopporn Suppipat to intimidate a creditor, police said.

Nopporn, 43, is founder and vice chairman of the privately held Wind Energy Holding Co and one of Thailand's 50 wealthiest people, according to Forbes.

Police said an arrest warrant had been issued for Nopporn, who is wanted for extortion, lese-majeste and other charges. His whereabouts are not known.

"Nobody in this company knows where he is, what he's doing or what all of this is about," Aman Lakhaney, a director of Wind Energy Holding, told Reuters.

Thailand's laws protecting the monarchy are among the strictest in the world. It is a crime to defame, insult or threaten the king, queen, heir-apparent or regent and anyone convicted of doing so faces up to 15 years in prison.